This invention relates to a manganese-zinc ferrite and a method for preparing the same, and more particularly, to a manganese-zinc ferrite having a high initial magnetic permeability xcexci suitable for use as the core of broad-band transmission transformers and a method for preparing the same.
In order for broad-band transmission transformers, for example, pulse transformers to ensure accurate digital communication, there is a need for core-forming manganese-zinc ferrite having a high broad-band permeability and exhibiting a high permeability throughout the region of 10 to 500 kHz.
To meet such a demand, the applicant provided in JP-A 6-204025 a manganese-zinc ferrite having a high broad-band permeability and exhibiting a high permeability throughout the region of 10 to 500 kHz. The manganese-zinc ferrite disclosed in this patent publication is a manganese-zinc ferrite containing 50 to 56 mol % calculated as Fe2O3 of iron oxide, 22 to 39 mol % calculated as MnO of manganese oxide, and 8 to 25 mol % calculated as ZnO of zinc oxide, which has been fired after adding thereto up to 800 ppm calculated as Bi2O3 of a bismuth oxide component and up to 1,200 ppm calculated as MoO3 of a molybdenum oxide component.
The manganese-zinc ferrite disclosed in this patent publication exhibits a high initial permeability over a broad band as demonstrated by an initial permeability of at least 9,000, at least 9,000, and at least 3,000 at 25xc2x0 C. and 10 kHz, 100 kHz, and 500 kHz, respectively.
In order for pulse transformers as described above to realize size reduction and fast communication, it is important that the ferrite exhibits a higher initial permeability especially in a low-frequency region of about 10 kHz. This enables to sharpen the rise of output pulses and reduce the working attenuation even when the number of windings is reduced, ensuring accurate digital communication.
It is, therefore, an object of the present invention to provide a manganese-zinc ferrite exhibiting a high initial permeability over a broad band and especially in a low-frequency region of about 10 kHz and a method for preparing the same.
(1) A manganese-zinc ferrite comprising 50 to 56 mol % calculated as Fe2O3 of iron oxide, 22 to 39 mol % calculated as MnO of manganese oxide, and 8 to 25 mol % calculated as ZnO of zinc oxide as main components, and having a mean crystal grain size of more than 50 xcexcm to 150 xcexcm.
(2) The manganese-zinc ferrite of (1) which contains, based on the main components, up to 800 ppm calculated as Bi2O3 of a bismuth oxide component and up to 1,200 ppm calculated as MoO3 of a molybdenum oxide component as auxiliary components.
(3) The manganese-zinc ferrite of (1) or (2) further containing 50 to 500 ppm calculated as CaO of calcium oxide.
(4) The manganese-zinc ferrite of any one of (1) to (3) which has a magnetic permeability xcexc100 as measured at a frequency of 10 kHz and a magnetic flux density B of 100 millitesla and a magnetic permeability xcexc1 as measured at a frequency of 10 kHz and a magnetic flux density B of 1 millitesla, the permeability xcexc100 being at least 1.20 times the permeability xcexc1.
(5) The manganese-zinc ferrite of any one of (1) to (4) which has an initial magnetic permeability xcexci of at least 15,000 at 10 kHz.
(6) The manganese-zinc ferrite of any one of (1) to (5) which has experienced at least one thermal ramp-down step during firing.
(7) A method for preparing a manganese-zinc ferrite by firing, characterized in that the firing includes a main temperature holding step at 1,200 to 1,450xc2x0 C. and a thermal ramp-down step prior to the main temperature holding step, and the lowest temperature reached by the mid-firing thermal ramp-down step is set in the range of 1,000 to 1,400xc2x0 C. and lower by at least 50xc2x0 C. than the hold temperature of the main temperature holding step.
(8) The method for preparing a manganese-zinc ferrite of (7) wherein the manganese-zinc ferrite of any one of (1) to (6) is obtained.